This invention relates to an improved process for the preparation of low molecular polyalcohols. This process comprises an alkaline two-stage catalytic hydrogenation of a mixture of various low molecular weight hydroxyaldehydes, hydroxyketones and polyhydric alcohols which is obtained from the autocondensation of formaldehyde (such a mixture will hereinafter be referred to as "formose"). The invention also relates to the use of these polyalcohols for the production of polyurethanes.
Since the work of Butlerow and Loew (Ann. 120, 295 (1861) and J. prakt, Chem. 33, 321 (1886)) in the previous century it has been known that hydroxyaldehydes, hydroxyketones and polyhydric alcohols are formed when the autocondensation of formaldehyde hydrate (formose synthesis) is carried out under the influence of basic compounds such as calcium or lead hydroxide. Studies on the synthesis of formose have repeatedly been carried out since then. References in this connection include Pfeil, Chem. Berichte 84, 229 (1951), Pfeil and Schroth, Chemische Berichte 85, 303 (1952), R. D. Partridge and A. H. Weiss, Carbohydrate Research 24, 29-44 (1972), Formoses from Glyceraldehyde and Dihydroxyacetone according to Emil Fischer, German Pat. Nos. 822,385, 830,951 and 884,791, U.S. Pat. Nos. 2,121,981, 2,224,910, 2,269,935 and 2,272,378 and British Pat. No. 513,708. All these processes known in the art have certain disadvantages (poor volume/time yields, discolored by-products), but new processes have recently been developed, by which substantially colorless formoses free from undesirable by-products can be produced in high yields with the aid of conventional catalysts.
One of these new processes consists of carrying out the condensation of formaldehyde hydrate in the presence of catalysts consisting of soluble or insoluble lead(II) salts or of lead (II) ions bound to high molecular weight carriers and in the presence of formose as a cocatalyst.
The reaction temperature employed is generally in the range of from 70.degree. to 110.degree. C., and preferably from 80.degree. to 100.degree. C. The pH of the reaction solution is adjusted by the controlled addition of an inorganic and/or organic base to a value of from 6.0 to 8.0, and preferably from 6.5 to 7.0 up to a conversion of from 10 to 60%, and preferably from 30 to 50%. Thereafter, the pH is adjusted to a value of from 4.0 to 6.0, and preferably from 5.0 to 6.0. It was surprisingly found that this special pH control and subsequent cooling at different residual formaldehyde contents (from 0 to 10% by weight, and preferably from 0.5 to 6.0%, by weight) enables the proportion of products obtained to be varied in a reproducible manner.
When the autocondensation of formaldehyde hydrate has been stopped by cooling and/or by deactivation of the lead-catalyst by means of acids, the catalyst, and optionally also the water contained in the products is removed. Further details of these processes may be found in German Offenlegungsschriften Nos. 2,639,084 and 2,732,077.
Another possibility of obtaining highly concentrated, colorless formoses in high volume/time yields consists of a process described in German Offenlegungsschrift No. 2,714,084. In this process, aqueous formalin solutions and/or paraformaldehyde dispersions are condensed in the presence of a soluble or insoluble metal catalyst and a cocatalyst which has been obtained by the partial oxidation of a dihydric or higher hydric alcohol having a molecular weight of from 62 to 242 and containing at least two adjacent hydroxyl groups or a mixture of such alcohols. In this process, the pH of the reaction solution is controlled by the controlled addition of a base so that it is maintained at from 6.0 to 9.0 up to a conversion of from 5 to 40% and is then adjusted to from 4.5 to 8.0 until the condensation reaction is stopped. In the second stage, the pH is lower by from 1.0 to 2.0 units than in the first stage of the reaction. The reaction is stopped by the deactivation of the catalyst when the formaldehyde content has fallen to a level of from 0 to 10% by weight, and the catalyst is subsequently removed.
High quality formoses can also be obtained by the condensation of formaldehyde in the presence of a metal catalyst and more than 10% by weight, based on the formaldehyde, or one or more divalent or higher valent low molecular weight alcohols and/or higher molecular polyhydroxyl compounds (see German Offenlegungsschrift No. 2,714,104).
It is particularly economical to produce formoses directly from formaldehyde-containing synthesis gases according to German Offenlegungsschrift No. 2,721,093, i.e., without going through the stages of preparation of aqueous formalin solutions or paraformaldehydes. For this purpose, synthesis gases such as are obtained from the large scale production of formaldehyde are introduced continuously or intermittently, at a temperature of from 10.degree. to 150.degree. C., into an absorption liquid which is at a pH of from 3 to 10. This absorption liquid consists of water, monohydric or higher hydric low molecular weight alcohols and/or higher molecular polyhydroxyl compounds and/or compounds capable of enediol formation as cocatalysts and/or soluble or insoluble metal compounds optionally bound to high molecular weight carriers as catalysts. The formaldehyde is directly condensed in situ in the absorption liquid (optionally also in a following reaction tube or cascade of stirrer vessels). Autocondensation of formaldehyde is stopped by cooling and/or deactivation of the catalyst with acids when the residual formaldehyde content in the reaction mixture is from 0 to 10% by weight, and the catalyst is finally removed.
For various applications, the mixtures of hydroxyaldehyde, hydroxyketones and polyalcohols obtained by the process described above or known art processes must be converted into mixtures of polyalcohols by reduction of the carbonyl groups. (Such polyol mixtures obtained by the reduction of formoses will hereinafter be referred to as "formitols".) The reduction of formose may, for example, be carried out in aqueous solution at room temperature using sodium borohydride (see R. D. Partridge, A. H. Weiss and D. Todd, Carbohydrate Research 24 (1972), 42). On the other hand, it may be carried out using an electrochemical method.
Many processes for the catalytic hydrogenation of sugars as well as of formoses are known. Very different quantities and types of catalysts are used, depending on the process. Thus L. Orthner and E. Gerisch (Biochem. Zeitung 259, 30 (1933)) describe a process for the catalytic hydrogenation of formose in which a 4% aqueous formose solution is hydrogenated using 170%, by weight of Raney nickel, based on the quantity of formose, in a reaction lasting for from 7 to 8 hours at a temperature of 130.degree. C. and a hydrogen pressure of 120 bar. Such a process is, of course, economically unsatisfactory.
In U.S. Pat. No. 2,269,935, there has been disclosed a process in which a solution containing approximately 40% by weight of formose is hydrogenated at an acidic pH using 20% by weight of a nickel catalyst at a hydrogen pressure of from 600 to 620 bar and at a temperature of 120.degree. C. The disadvantages of this variation of the process are not only the high operating pressure but also the low pH value necessary, which results in green colored products due to nickel ions.
In U.S. Pat. No. 2,224,910, there is disclosed a process for the hydrogenation of formose in which a 40% formose solution is hydrogenated using 30% by weight of Raney nickel, based on formose, at a hydrogen pressure of from 140 to 210 bar and at pH of 7 for 4 hours. This process again is unsatisfactory due to the large quantity of catalyst used and the long reaction time required.
Other hydrogenation processes have been described in German Pat. Nos. 705,274, 725,842, 830,951, 888,096 and 1,044,157 and in U.S. Pat. Nos. 2,271,083, 2,272,378, 2,276,192, 2,760,983 and 2,775,621. All these processes have one or more of the following disadvantages: large and expensive apparatus, difficulty of handling due to high hydrogen pressures, high catalyst consumption based on the quantity of hydrogenated product (from 10 to 200% by weight) and colored products due to long hydrogenation times (from 1 to 10 hours). Common also to all the known hydrogenation processes is the use of metal catalysts and in some cases noble metal catalysts. It is particularly common to use Raney nickel. As is known in the case, Raney nickel only develops its full activity in the alkaline pH range. Since formose tends to undergo caramelization in an alkaline medium and heavily discolored products are formed, the processes known in the art are generally not carried out at an alkaline pH but in a slightly acidic to neutral pH range.
According to an earlier proposal by the present Applicants (U.S. application Ser. No. 965,645, filed Dec. 1, 1978), formose solutions (optionally as mixtures with other natural and/or synthetic sugars) can be rapidly hydrogenated in a highly alkaline medium with a low consumption of catalyst at a hydrogen pressure of from 100 to 200 bar and at a temperature of from 50.degree. to 250.degree. C. to form colorless solutions of polyol mixtures. In the process according to U.S. application Ser. No. 965,645, a formose solution at a concentration of more than 20% is pumped batchwise into a reactor which is at a temperature of from about 100.degree. to 200.degree. C. at a rate such that the concentration of the groups which are to be reduced does not rise above 2% by weight. Hydrogenation is carried out at a pH value of from 7.5 to 12.5 with a catalyst consumption of from 10.sup.-4 to 5.times.10.sup.-2 % by weight (based on formose) and at a hydrogen pressure of from 50 to 300 bar. The catalysts used are, in particular, metals having an atomic number of from 23 to 29 (and especially Raney nickel).
When the formitol solutions obtained by the known hydrogenation processes are concentrated, for example, by thin layer distillation in a vacuum, troublesome yellow discolorations of hitherto unknown cause generally occur, regardless of the particular process used for the preparation of the formose solution and the conditions of hydrogenation.